Not far away was another tunnel - this time it started with a gentle incline so we could get inside.

As it went further and further back, we saw the marks of the tools used to bore through the rock, before the tunnel dropped away deep into the ground.

"It is clear people are digging to find archaeological treasures," says Dr Osama al-Shimi, the archaeologist in charge of the site.

"People dream when they dig they'll especially find gold, and get rich quick," he says.

But Dr al-Shimi says it is dangerous to tackle the people responsible because they often have weapons.

Lunar landscape

The illegal digging is not just happening at Giza.

In fact, an hour's drive from Cairo, in Dahshur - close to the site of the famous Bent Pyramid of the Pharaoh Sneferu - some areas looked like a cratered lunar landscape because of the concentration of holes that had been dug.

Gunmen had also attacked storehouses at other sites close by, at Saqqara and Abusir.

They held the antiquities from ongoing excavations but, because they had not yet been registered and were still being studied, no one knows just how many were stolen in the raids.

Our search into the extent of the problem took us to southern Egypt, too. To Luxor, once the great ancient capital city known as Thebes.

Here, near the Valley of the Kings and the many wondrous temples, the ground is saturated with archaeological riches.

But over the past two years the police have been inundated with audacious cases of people tunnelling for antiquities.

The security forces showed us extraordinary video footage they had taken of vast tunnel networks that have been discovered.

In Luxor, the digging often starts within a compound close to the ancient sites or even inside a home to make detection harder.

Media captionKent Weeks, Professor of Egyptology at the American University in Cairo, says the problem is worse than ever

"Illegal digging has always happened but it increased since the revolution," says Brig Hosni Hussain, the head of tourism and antiquities police in Luxor. "They think there is no security."

But the police chief played down the problem, dismissing the possibility that there may still be many undiscovered tunnels.

He also insisted that everything that had been stolen to date had been recovered.

"We know what's going on," says Brig Hussain. "And I am pleased to tell you that in the end, because of our work, nothing has been stolen anywhere in the country."

Black market boom

Since the revolution and the decline of the ability of central government to control the sites, it does seem to have become a bigger problem than ever beforeKent Weeks, Professor of Egyptology

Unfortunately, that is not the case. And it was not difficult to prove it.

The unsanctioned excavation and trade of ancient artefacts are illegal in Egypt. As a result there are many black market dealers in antiquities in Cairo.

A local colleague went to meet a well-known dealer, saying he was representing a British buyer who was an Egyptology expert.

The dealer trusted him enough to produce a crate of what he said were artefacts more than 3,000 years old, from Middle Kingdom Egypt.

Most of them appeared to be statues of servant figures often found in tombs.

The dealer said that as long as his face was not shown, video footage of the antiquities could be taken.

He said an archaeologist could be brought to verify the pieces at a subsequent meeting and that his starting price for the whole batch was just $5,000 (£3,300).

Of course tomb robbery has been going on since ancient times, but Kent Weeks, Professor of Egyptology at the American University of Cairo says the past two years have seen a dramatic increase in such crimes.

"Since the revolution and the decline of the ability of central government to control the sites, it does seem to have become a bigger problem than ever before," he says.

Heritage damage

Professor Weeks believes it is, in part, down to economic reasons.

Over the past decade, Egyptians have come to understand the financial potential of archaeological finds, he thinks. And since the revolution, police have been unable to do their job effectively.

A decline in law and order has been very apparent across the country, with the security forces having lost both the fear and the respect that they had elicited before the events of 2011.

Professor Weeks believes that in this environment serious lasting damage is being done to Egyptian heritage, and indeed world heritage.

"Perhaps thousands of objects have been stolen. We simply don't know," he says.

"Those objects are going to be preserved and protected by the thieves, because they make money from them, and also by the collectors as they want them in their collection. So the objects are safe."

But Professor Weeks says that what is really going missing by the day is the archaeological and historical context of the artefacts - which he believes is even more valuable than the pieces themselves.

"Where did it come from? What tomb was it found in? What other objects were accompanying it? This is the kind of information that is completely and forever lost, the moment an object is stolen."

People living by the pyramids in Giza told us new tunnels were still appearing all the time, and that on occasions they had even seen trucks parking close to the tunnel entrances in the desert.

In spite of assurances from the Egyptian authorities that armed guards were monitoring the sites and ensuring no more illegal digging could take place, we saw no evidence of that.